1. Field of the Invention
The present invention relates to a sheet punching device that is integrally provided on or annexed to, or separately provided to an image forming apparatus such as a printer, a copying machine, and a facsimile and that performs punching on a recording medium (sheet) discharged from the image forming apparatus, and an image forming system provided with the sheet punching device.
2. Description of the Related Art
A punching unit that punches sheets one by one is currently used, as a unit to punch sheets conveyed from an image forming apparatus, in a post-processing apparatus. The punching unit of this type punches the conveyed sheets one by one, and is advantageous in terms of load required for punching being reduced and its productivity.
When the position of a sheet at the time of punching is not good, for example, badly skewed or lateral registration being badly displaced, the position of punched hole is misaligned, and thus alignment accuracy of the hole deteriorates. To prevent this, a method has been developed in which the skew of a sheet is corrected and punching is performed by further reading an edge portion on one side of the sheet in a direction orthogonal to a conveying direction of the sheet and by correcting the deviation based on that information so as to improve accuracy of the hole position.
More specifically, a punching device according to the invention disclosed in Japanese Patent No. 3363725 includes a punching unit that is movable in the direction intersecting the conveying direction of sheets and is provided with a puncher for punching a sheet in a direction intersecting the conveying direction of the sheet, a lateral registration detector to detect an edge portion of the sheet on one side parallel to the conveying direction of the sheet conveyed to the punching unit, and a moving unit for moving a punching position of the punching unit in the direction intersecting the sheet conveying direction based on positional information of the side edge portion of the sheet detected by the lateral registration detector. In this device, the lateral registration detector is moved in advance to near the side edge portion of the sheet based on size information of the sheet conveyed, and detects the side edge portion of the sheet on one side parallel to the conveying direction of the sheet conveyed. The punching unit is moved to a punching position based on the positional information of the side edge portion of the sheet. Accordingly, the punching position of the sheet can be aligned with the edge portion on one side of the sheet. Furthermore, even when an apparatus that outputs sheets conveys sheets in a laterally displaced manner, lateral deviation is continuously detected by the lateral registration detector and the punching unit is moved to an appropriate punching position. The sheets can be punched continuously without compromising the processing speed (image forming speed) of a sheet output apparatus, for example, an image forming apparatus.
In the invention disclosed in Japanese Patent No. 4478595, a sheet punching device that punches sheets and is mounted in a post-processing apparatus for conveying the sheets in a center-based manner includes a detector that detects a sheet position, a correcting unit that corrects lateral deviation in the sheet position, and a controller. The correcting unit includes a punching unit that is movable in a width direction of sheet. The controller controls the first sheet to be punched at a reference position in the center-based conveyance and controls the second and subsequent sheets to be punched by detecting a difference in lateral deviation from the first sheet with reference to the sheet position of the first sheet detected by the detector and by moving the punching unit for the extent of the difference such that the positions of holes on the sheets overlap with one another regardless of the positional deviation of the sheets themselves. Accordingly, the punching can be performed at the same position of each sheet although it may not be at the center of the sheet, and the accuracy of punching holes of sheet can be increased.
The technology to improve accuracy of the hole position in which the skew of a sheet is corrected and punching is performed by further reading an edge portion on one side of the sheet in a direction orthogonal to the sheet conveying direction and by correcting the deviation based on that information is already known.
Sheet skew and lateral registration may arise from how sheets are set on a paper feed tray. When the sheets are set on the near side than the center of the paper feed tray, the sheets are displaced towards the near side. When sheets are set on the back side than the center of the paper feed tray, the sheets are displaced towards the back side. Deviation occurs in the same direction for each sheet. Furthermore, curling and shrinkage of sheets, and sheet skew and lateral registration attributable to an assembling condition of a machine tend to occur in the same direction on the same machine.
In the technology disclosed in Japanese Patent No. 3363725, the detector is moved in advance to near the sheet edge for each sheet based on the sheet size (width) information notified from the image forming apparatus. When the actual sheets are continuously displaced in a direction far from the detector than the center of layout, it is necessary to move the detector for a useless distance each time, whereby the power to move the lateral registration detector is wasted.
Furthermore, in the technology disclosed in Japanese Patent No. 4478595, the first sheet is punched as the sheet is assumed to be positioned at the center of the layout regardless of its actual position, and the second and subsequent sheets are punched by correcting only for the extent of deviation from the first sheet in a main scanning direction. When the actual sheet position is largely displaced from the center of the layout, the punching position is displaced from the center of the sheet, whereby the sheets may not look good and the edges of the sheets may stick out from a file when filed.
There is needed to provide a sheet punching device in which, without causing an increase in the size and complexity of the device or compromising the output speed of sheets (speed of the image forming process), punching can be performed by aligning the hole position with respect to a side edge of a sheet and the power to move the lateral registration detector is not wasted as much as possible, and an image forming system provided with the sheet punching device.